start_date

sets the start value of the time scale

Date|undefined start_date;

Example

gantt.config.start_date = new Date(2018, 08, 10);
gantt.config.end_date = new Date(2018, 08, 20);
 
gantt.init("gantt_here");

Details

To apply the start_date option, you must use it in pair with the end_date one.

If both the start_date and end_date options are specified and you create a task that is outside the range, the task will disappear from the chart.
Optional parameters of the init method can be used as initial values of start_date and end_date.
start_date and end_date overwrite fit_tasks. If you want to use these settings together, you'll need to manage the time scale from code.

In this case we can extend the range:

gantt.attachEvent("onLightboxSave", function(id, task, is_new){
 var taskStart = task.start_date;
 var taskEnd = task.end_date;
 var scaleStart = gantt.config.start_date;
 var scaleEnd = gantt.config.end_date;
 
 // if the task is out of the range
 if(scaleStart > taskEnd || scaleEnd < taskStart ){
  // update the time scale range
  gantt.config.end_date=new Date(Math.max(taskEnd.valueOf(), scaleEnd.valueOf()));
  gantt.config.start_date=new Date(Math.min(taskStart.valueOf(),scaleStart.valueOf()));
  gantt.render();
 }    
 return true;
});

Or add validation to the lightbox control:

gantt.attachEvent("onLightboxSave", function(id, task, is_new){
    var taskStart = task.start_date;
    var taskEnd = task.end_date;
    var scaleStart = gantt.config.start_date;
    var scaleEnd = gantt.config.end_date;
 
    // check if the task is out of the range
    if(scaleStart > taskEnd || scaleEnd < taskStart ){
        gantt.message({
            type:"warning", 
            text:"Warning! The task is outside the date range!",
            expire:5000
        });
          return false;
    } 
    return true;
});